Uterine sensitivity for ovo-implantation is studied by examining fluctuations of morphological, biochemical and biological parameters in the uterus at various stages of early pregnancy. Effects of steroid hormones, synthetic compounds and antiserum to deciduoma on the above- mentioned parameters are studied. Physiological refractoriness of the uterus to stimulation of blastocyst or to mechanical stimulation for decidual reaction is characterized and will be attempted to produce artificially in order for the development of new contraceptive methods. Ovarian function in relation to uterine sensitivity is also studied by determination of ovarian steroid hormones levels in animals under various hormonal conditions and uterine conditions.